When exposed to radiation such as X-rays, an energy-storing phosphor (stimulable phosphor, which gives off stimulated emission) absorbs and stores a portion of the radiation energy. The phosphor then emits stimulated emission according to the level of the stored energy when exposed to electromagnetic wave such as visible or infra-red light (i.e., stimulating ray). A radiation image recording and reproducing method utilizing the energy-storing phosphor has been widely employed in practice. In that method, a radiation image storage panel, which is a sheet comprising the energy-storing phosphor, is used. The method comprises the steps of: exposing the storage panel to radiation having passed through an object or having radiated from an object, so that radiation image information of the object is temporarily recorded in the panel; sequentially scanning the panel with a stimulating ray such as a laser beam to emit stimulated light; and photoelectrically detecting the emitted light to obtain electric image signals. The storage panel thus treated is subjected to a step for erasing radiation energy remaining therein, and then stored for the use in the next recording and reproducing procedure. Thus, the radiation image storage panel can be repeatedly used.
The radiation image storage panel (often referred to as energy-storing phosphor sheet) used in the radiation image recording and reproducing method has a basic structure comprising a support and a phosphor layer provided thereon. However, if the phosphor layer is self-supporting, the support may be omitted. Further, a protective film is normally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical shock.
Phosphor layers of various kinds are known. Examples of the known phosphor layers include a phosphor layer comprising a binder and an energy-storing phosphor dispersed therein, a phosphor layer which is formed by a gas phase-accumulation method or by a firing method and which comprises agglomerate of an energy-storing phosphor without binder, and a phosphor layer comprising energy-storing phosphor agglomerate impregnated with a polymer material.
A variation of the radiation image recording and reproducing method is known. While an energy-storing phosphor of the storage panel used in the conventional type plays both roles of radiation-absorbing function and energy-storing function, those two functions are separated in the method. In the method, a radiation image storage panel comprising an energy-storing phosphor (which stores radiation energy) is used in combination with a phosphor screen comprising another phosphor (radiation-absorbing phosphor) which absorbs radiation and emits ultraviolet or visible light. The disclosed method comprises the steps of: causing the radiation-absorbing phosphor of the screen to absorb and convert radiation having passed through an object or having radiated from an object into ultraviolet or visible light; causing the energy-storing phosphor of the panel to store the energy of the converted light as radiation image information; sequentially scanning the panel with a stimulating ray to emit stimulated light; and photoelectrically detecting the emitted light to obtain electric image signals. The present invention can be also applied to the radiation image storage panel used in this type of the method.
The radiation image recording and reproducing method (or radiation image forming method) has various advantages as described above. Even so, however, it is still desired that the radiation image storage panel used in the method have as high sensitivity as possible and, at the same time, give a reproduced radiation image of high quality (in regard to sharpness and graininess).
In order to improve the sensitivity and the image quality, it is proposed that the phosphor layer of the storage panel be prepared by a gas phase-accumulation method such as vacuum vapor deposition or sputtering. The process of vacuum vapor deposition, for example, comprises the steps of: heating to vaporize an evaporation source comprising a phosphor or materials thereof by means of a resistance heater or an electron beam, and depositing and accumulating the vapor on a substrate such as a metal sheet to form a layer of the phosphor in the form of columnar crystals.
The phosphor layer formed by the gas phase-accumulation method contains no binder and consists of the phosphor only, and there are cracks among the prismatic crystals of the phosphor. Because of the cracks, the stimulating ray can stimulate the phosphor efficiently and the emitted light can be collected efficiently, too. Accordingly, a radiation image storage panel having the phosphor layer formed by the gas phase-accumulation method has high sensitivity. At the same time, since the cracks prevent the stimulating ray from diffusing parallel to the layer, the storage panel can give a reproduced image of high sharpness.
When excited with, for example, a stimulating ray, the phosphor layer of the panel gives off emission (stimulated emission) accompanied by afterglow, which is emitted from the phosphor layer consecutively after the stimulated emission. Since causing noises to lower the S/N ratio in reading the radiation image information, the afterglow is preferably made as weak as possible in consideration of the image quality such as sharpness. The afterglow of the phosphor is a serious problem particularly in the case where the emission from the panel is detected in a point-scan detecting system with a photomultiplier since the scanning speed in that case is very high.
JP-A-4-240600 discloses a process for preparation of a radiation image storage panel giving stimulated emission accompanied by a small amount of afterglow. In the process, a layer of alkali metal halide stimulable phosphor such as RbBr:Tl or matrix thereof is formed by a gas phase-accumulation method such as the vapor deposition process, and then is subjected to heat treatment under an atmosphere containing an activator and an oxygen compound.
JP-A-62-156191 discloses improvement of the stimulated emission afterglow. In the Publication, powdery alkali metal halide stimulable phosphor such as RbBr:Tl is fired together with an oxygen-containing compound, so that the obtained phosphor contains oxygen to give stimulated emission improved in the afterglow.
WO 01/03156A1 discloses a CsX:Eu stimulable phosphor and a phosphor screen comprising a phosphor layer formed by a gas phase-accumulation method. In the publication, a film comprising the CsX:Eu stimulable phosphor layer formed by the vapor-deposition process is excited with a ultraviolet light (wavelength: 280 nm), and thereby emitted instant emission is observed. The instant emission spectrum shown in the publication has a sharp emission peak at approx. 440 nm, but it is not clear whether another emission peak is observed at approx. 490 nm or not. The publication is silent with respect to a peak at approx. 490 nm.